Continuous-tone images, such as charts, drawings, and pictures, may be represented as a two-dimensional matrix of picture elements (pixels). The spatial resolution and intensity level for each pixel are chosen to correspond to the particular output device used. Typically, digital halftoning is used to transform a continuous-tone image into the desired matrix of pixels.
Conventional methods for digital halftoning generally fall into two categories: clustered dot and dispersed dot. As is well known in the art, in clustered dot techniques, the size of the printed dot is varied to control the perceived gray level or, equivalently, density of the printed tone. These methods may be thought of as amplitude modulation (AM) halftoning techniques since the amplitude or size of the dots controls the printed gray level.
FIG. 1 is a block diagram illustrating a conventional clustered dot halftoning process. As shown in FIG. 1, typically, an image x(m, n) is transformed, or tone compensated, at block 2, so that the printed gray level for each input value is correct. Tone compensation (block 2) is used to correct for attributes of the halftoning algorithm, image calibration, and printer behavior. In particular, most printers are not linear due to the inevitable effects of dot overlap. The dot size is then modulated at block 4 and the resulting signal is sent to the printing system. Typically, dot size modulation is performed using screening, which uses a threshold array that is compared to the pixel value. AM halftoning methods have certain advantages such as increased robustness to printing artifacts. However, a limitation of AM halftoning methods is that the choice of dot size presents a fundamental tradeoff between spatial resolution and number of gray levels.
Dispersed dot halftoning methods control the printed gray level through the spacing or, equivalently, the frequency of dot placement. Dispersed dot halftoning may be thought of as a frequency modulation (FM) halftoning technique since the frequency or spacing of the dots controls the printed gray level. Dispersed dot halftoning methods are well known, and include, for example, error diffusion halftoning, screening and, most recently, iterative search based halftoning. FIG. 2 is a block diagram illustrating a conventional dispersed dot halftoning process. As with the clustered dot halftoning process, the image x(m, n) is transformed, or tone compensated at block 6, so that the printed gray level for each input value is correct. The spacing of the dots is then appropriately dispersed at block 8 and the resulting signal is sent to the printing system.
Typically, dispersed dot halftoning uses the smallest dots possible to print as it is undesirable to actually notice the dots in the printed image. However, a drawback of dispersed dot halftoning is that the printer must be capable of printing well formed isolated dots. For example, error diffusion is typically used only in inkjet type printers because they generally can print stable isolated dots. However, error diffusion is typically not used in commercial electrophotographic printers and copiers, such as laser printers, due to their instability in producing binary or multilevel halftones.